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Transcript of Adsorption/desorption behavior of cationic dyes on ... · BM and GM are observed to be 40, 64 and...
JMES, 2017, 8 (3), pp. 1082-1096 1082
JMES, 2017 Volume 8, Issue 3, Page 1082-1096
http://www.jmaterenvironsci.com /
1. Introduction
The textile industry plays an important role in the global economy of our life. It consumes large quantities of
water both for finishing, fixing dyes on the substrate, and generating large amounts of waste waters, thus, the
discharged effluents are highly altered by dyes [1].Effluent treatment from dyeing and finishing processes in the
textile industry is one of the most significant environmental problems [2]. Therefore, color removal from
wastewater is very important for solving the ecological, biological and industrial problems associated with the
dyes [3]. In effect, its oral consumption is toxic, hazardous and carcinogenic due to presence of nitrogen. It
when discharged into receiving streams will affect the aquatic life and causes detrimental effects in liver, gill,
intestine, gonads and pituitary gonadotrophic cells. In humans, it may cause irritation to the respiratory tract if
inhaled and causes irritation to the gastrointestinal tract upon ingestion. Contact to cationic dyes as BR46 with
skin cause irritation with redness and pain. Upon contact with eye will lead to permanent injury of human eyes
and laboratory animals [4, 5].
Malachite green (GM) is a N-methylated diaminotriphenylmethane dye, which is most widely used for coloring
purpose, amongst all other dyes of its category. This cationic dye is generally used for the dyeing of cotton,
wool, silk, leather, jute, paper, and also widely used in distilleries for coloring purposes. It is also used as
therapeutic agent (fungicide, ectoparasiticide) and as antiseptic, but only for external applications on the wounds
and ulcers, however, its oral consumption is toxic, hazardous and carcinogenic due to presence of nitrogen. It
when discharged into receiving streams will affect the aquatic life and causes detrimental effects in liver, gill,
kidney, intestine, gonads and pituitary gonadotrophic cells. In humans, it may cause irritation to the respiratory
tract if inhaled and causes irritation to the gastrointestinal tract upon ingestion. Contact to malachite green with
skin cause irritation with redness and pain. Upon contact with eye will lead to permanent injury of human eyes
Journal of Materials and Environmental Sciences ISSN : 2028-2508
Copyright © 2017,
http://www.jmaterenvironsci.com/
Adsorption/desorption behavior of cationic dyes on Moroccan clay: equilibrium
and mechanism
A. Bennani Karim*1,2, B. Mounir
1, M. Hachkar1, M. Bakasse
3, A. Yaacoubi2
1The team of research Analysis, Checks and Environment, High School of Technology, University Cadi Ayyad, BP 89, Safi,
Morocco 2The team Environmental and Experimental Methodology, Laboratory of Applied Organic Chemistry, Faculty of Sciences
Semlalia, PO Box 2390, Marrakech, Morocco. 3The team of Analysis of the Microphones Polluting Organic, Faculty of the Sciences, University Chouaib Doukkali, BP 20, El
Jadida, Morocco.
Abstract
Moroccan clay were prepared and tested as adsorbent for removal of three basic dyes:
Basic Red 46 (BR46), Methylene Blue (BM) and Green Malachite (GM) from aqueous
solution. The adsorption kinetic was studied, the experimental data isotherms were
analysed using the Langmuir, Freundlich and Redlich Peterson models.
Thermodynamical parameters (∆H°, ∆S°, ∆G°, and Ea were also calculated for the dyes
adsorption onto clay, it revealed that the adsorption process is exothermic in nature for
the BR46 dye. Nevertheless, it is endothermic for BM and GM. The Ea values for BR46,
BM and GM are observed to be 40, 64 and 76 kJ/ mole, respectively. These results
indicates that the adsorption has a low potential barrier for BR46 dye and assigned to a
physisorption. Whereas, the chemical adsorption mechanism is occurred for the BM and
the GM dye. The desorption of the basic dyes using NaCl and CaCl2 at different salt
dyes ratio onto raw clay samples were studied using a batch adsorption. Results showed
that the presence of other ions (Na+ and Ca
2+) reduces the dyes uptake onto clay and
promote significatively the dyes desorption when the salt dye ratio is less than 1,
Whereas, the desorption effect is limited when this ratio is more than 1. The adsorption
mechanism has been confirmed by IR spectra decomposition.
Received 01Jul 2016,
Revised 09Nov 2016,
Accepted 14 Nov 2016
Keywords
Basic dyes, Moroccan clay, Adsorption, desorption, thermodynamic
parameters, IR spectrum.
JMES, 2017, 8 (3), pp. 1082-1096 1083
and laboratory animals [5]. It is known to be highly toxic to mammalian cells and to act as a tumour enhancing
agent. However, despite the large amount of data on its toxic effects, MG is still used in aquaculture and other
industries [5- 7]. Methylene blue dye is the most commonly used in the dyeing of cotton, silk and wood. It can
create eye burns causing permanent damage to the eyes of man and animals. Its inhalation can lead to breathing
difficulties by ingestion through the mouth which produces a burning sensation, nausea, vomiting, abundant and
cold sweat [8]. In order to minimize the possible damages to human and environment arising from the effluents
containing this dyes, various kinds of treatments technologies such as adsorption, chemical flocculation [9, 10],
chemical oxidation, froth flotation, ultra filtration and biological treatment technologies have been employed
[11]. ng these, adsorption is one of the popular methods for wastewater treatment on account of its easy and
inexpensive operation [3]. The most common adsorbents reported for basic dyes removal from wastewater are
perlite [6], chitosan bead [7] activated carbon [12], orange peel [13] and natural clay [14]. Clays are particularly
attractive adsorbents, since these are easily available at low cost [11]. In the present study, experiments have
been performed for the removal of BR46, BM and GM dyes using adsorption techniques. Moroccan crude clay
has been selected as adsorbent and added to the solution containing dye. The effects of temperature, initial dye
concentration, adsorption isotherms have been studied under stirred condition. The desorption efficiency was
investigated by adding NaCl and CaCl2 at different concentrations.
2. Materials and methods 2.1. Materiel
2.1.1. Adsorbate
Some characteristics of the basic cationic dyes used as adorbates are summarized in table 1
Adsorbat Masse
Molaire
(g.mole-1
)
Origine max
(nm)
Structure
BR46 357,5 SDI textile
company
(Safi,
Morocco)
532
GM 463,5 SDI textile
company
(Safi,
Morocco)
618
C N
CH3
CH3
N
CH3
CH3
Cl+
BM 319,5 High
School of
Technology
666 N
S NNCH
3
CH3
CH3
CH3
Cl+
CH3
N
N
N N
N
N
CH3
CH3
Table 1: Some characteristics of the basic cationic dyes used as adorbates
JMES, 2017, 8 (3), pp. 1082-1096 1084
2.1.2. Adsorbent materiel
The raw adsorbent Moroccan clay used in this work is collected from a natural basin in the region of Safi
(Morocco), crushed and sieved to < 0, 08 µm size fractions. Then, it was dried at 105 °C for 24h and used for
further experiments. The chemical composition of this clay is: 53,11 % SiO2, 16,95 % Al2O3, 5,94 % Fe2O3,
3,51 % CaO, 2,51 % MgO; 0,2 % SO3; 4,64 % K2O; 0,26% Na2O; 0,09 % P2O5 [15].
The mineralogical identification is performed by XRD in Siemens D500 diffractometer using a 106 CuKα
radiation (λ = 1.5406 Å) is produced under conditions of 40 kV and 20 mA, his predominant peaks are 9,99 Å;
7,16 Å; 4,25 Å; 3,03 Å and 2,9 Å which correspond to illite, kaolinite, quartz, calcite and dolomite [15].
Fourier transform infrared spectroscopy (FTIR) of the adsorbent was obtained (and transformed to Microsoft
Excel) by using an FTIR spectrophotometer (JASCO corp, FT/ IR 620 REV. 1.00.) to assess the different band
attribution. About 100 mg KBr disks containing approximately 1% of clay samples was prepared shortly before
recording the FTIR spectra in the range of 400–4000 cm-1
and with a resolution of 4 cm-1
. The resulting spectra
were the average of 30 scans.
2.2. Methods
2.2.1. Adsorption experiments
The adsorption experiments were carried out in stirred batch by varying dye concentration while the adsorbent
clay was kept constant to 40 mg, which was used as optimal amount for further experiments in our earlier
studies [15] in 100 mL dye solution. Samples of 3 mL of mixture were withdrawn from the batch at
predetermined time intervals and the supernatant was centrifuged for 8 min at 2800 r.p.m. The dyes
concentrations were determinate from their absorbance characteristics in the UV-Visible range. A
spectrophotometer (GBC (Ajax, Ontorio) UV/visible 911) was used for experiments. A linear correlation was
established between the dye concentration and the absorbance at λmax = 532; 666 and 618 nm for BR46, BM and
GM respectively in the range Cdye = 0 -28 mg/L with a coefficient of correlation r² ~ 1.
2.2.2. Sorption isotherms
The equilibrium isotherms are very important for understanding the adsorption systems. The variation of the dye
concentration method, while keeping the same mass of the adsorbent, is used to calculate the adsorption
characteristics of the adsorbent.
Preliminary experiments demonstrated that the equilibrium was established in 70 min for all the dyes. The
sorption isotherms were established using adsorbent clay suspensions in BR46, BM and GM solutions (solid /
solution ratio = 0.4 g/L used as optimal amount in our earlier studies [15] in a range from 10 to 28 mg/L at
natural pH of 6. The suspensions were stirred during the equilibrium time at room temperature ± 22°, and then
centrifuged. The dye concentration was determined as above.
2.2.3. Desorption studies
The desorption studies were carried out by studying the effect of the addition of salts (NaCl and CaCl2). 50 mL
of 0,003 M of NaCl and 0,003 M of CaCl2 was added separately to 50 mL samples containing 40 mg of the clay
fully loaded with initially 0,0006 M of BR46, BM and GM separately . In effect, this amount of sodium and
calcium electrolytes solutions were prepared and added to suspensions stirred until equilibrium with different
molecular ratio of salt and dyes: 0.25; 0.5; 1; 2 and 5 in separate glasses, The suspensions were agitated for 60
min, then the supernatants were collected for analyses as follow: Samples of 3 mL of mixture were withdrawn
from the batch at predetermined time intervals and the supernatant was centrifuged for 8 min at 2500 r.p.m. The
desorbed dye concentrations were determinate from their absorbance characteristics in the UV – visible range. A
spectrophotometer (GBC (ajax, ontorio) UV/visible 911) was used for experiments. The salts NaCl and CaCl2
used in these tests were of analytical grade, corresponding to a purity of 99%.
The adsorbent capacity is an important factor because it determines how much adsorbent is required to
quantitatively remove a specific amount of metal ions from the solutions [16]
The amount of dye ion desorbed was obtained from the difference between the amount of dye ion fully loaded
on the adsorbents and the amount of the same in solution. Reduced adsorption is calculated by the following
equation:
Desorption % = (q0 - qi / q0)*100 (1)
Were q0 is the maximum adsorption capacity in the absence of salts and qi the maximum adsorption capacity in
the presence of salts in a ratio R
JMES, 2017, 8 (3), pp. 1082-1096 1085
2.2.4. Effect of temperature:
Temperature effect on adsorption for determination of thermodynamic parameters was studied for four
temperatures: 30; 40; 50 and 60°C. A sample of 40 mg of clay was added to dyes solution (100 mL, 16 mg/l
for the GM, 20 mg/L for the BR46 and 24 mg/L for the BM) at pH = 6. The experiments were carried out in a
constant temperature shaker bath which controlled the temperature to within ±1°C.
Arrhenius equation has been applied to evaluate the activation energy of adsorption representing the minimum
energy that reactants must have for the reaction to proceed, as shown by following relationship:
Ln k2= ln A - Ea /RT
Where k2 is the rate constant obtained from the pseudo- second- order kinetic model, (g/mg. min), Ea is the
Arrhenius activation energy of adsorption, (kJ/mole), A is the Arrhenius factor, R is the universal gas constant
(8.314J/mole K) and T is the absolute temperature. When Ln k2 is plotted against 1/T, a straight line with slope
of - Ea/R is obtained [17].
3. Results and discussion 3.1. Characterization of adsorbent:
Table 2 shows the tabulated data for FTIR spectra band assignments for raw clay sample obtained from Fig. 1.
In the range of 400 - 4000 cm-1
. As shown in Figure 1, the spectra display a number of absorption peaks,
indicating the complex nature of the clay adsorbent examined. The FTIR spectroscopic analysis indicated broad
bands at 3498 cm-1
, representing bonded OH groups, while the bands at 3614 cm-1
is associated with the
stretching vibrations of hydroxyl groups which are coordinated to the octahedral magnesium and the tetrahedral
silicon [18].
The bands at 527 and 473 cm-1
corresponded to deformation vibrations of Si-O-Al and Al-OH, respectively. The
bands at 3415 and 1651 cm-1
are attributed to adsorbed water and zeolitic water. The OH bending bands appear
at 985 cm-1
[19].
The additional peaks at 1442 and 874 cm-1
, indicate the calcite CO group stretching vibration presence[20],
while those at 2526; 1817 and 1450 cm-1
are related to dolomite bands, associated with the stretching vibration
of the group CO32-
[20].
3.2. Adsorption isotherms
Adsorption isotherms describe qualitative information on the nature of the solute-surface interaction as well as
the specific relation between the concentration of adsorbate and its degree of accumulation onto adsorbent
surface at constant temperature.
Adsorption isotherms are critical in optimizing the use of adsorbents, and the analysis of the isotherm data by
fitting them to different isotherm models is an important step to find the suitable model that can be used for
design purposes [21]. Three models were tested to describe the adsorption experimental results: the Langmuir
model, the Freundlich model and Redlich- Peterson model (R-P) which contains three parameters including
features of Langmuir and Freundlich isotherm equations [22].
⧫ Langmuir isotherm.
In 1918, Langmuir proposed a theory to describe the adsorption of gas molecules onto metal surfaces [23].
Langmuir’s model of adsorption predicts the existence of monolayer coverage of the adsorbate at the outer
Figure 1:FTIR spectra of Safi Moroccan clay
JMES, 2017, 8 (3), pp. 1082-1096 1086
surface of the adsorbent. The isotherm equation further assumes that adsorption takes place at specific
homogeneous sites within the adsorbent, which implies that all adsorption sites are identical and energetically
equivalent.
Theoretically, the sorbent has a finite capacity for the sorbate. Therefore, a saturation value is reached beyond
which no further sorption can occur [24]. The saturated or monolayer capacity can be represented as the known
Langmuir equation:
qe = qmax *Kl *Ce/ 1+Kl*Ce (2)
Where qe is the equilibrium dye concentration on the adsorbent (mg g-1
), Ce is the equilibrium dye concentration
in solution (mg dm-3
), qmax is the monolayer capacity of the adsorbent (mg g-1
), and KL is the Langmuir
adsorption constant (dm3 mg
-1)
⧫Freundlich isotherm
The Freundlich equation [25] is an empirical equation employed to describe heterogeneous systems,
characterized by the heterogeneity factor 1/n, describes reversible adsorption, and is not restricted to the
formation of the monolayer,
qe = KFCe1/n
(3)
Where qe is the equilibrium dye concentration on adsorbent (mg g−1
), Ce is the equilibrium dye concentration in
solution (mg dm−3
), KF is Freundlich constant which indicates the sorption capacity of the sorbent. (mg g−1
), and
1/n is the heterogeneity factor.
⧫Redlich and Peterson isotherm
Redlich and Peterson incorporated three parameters into an empirical isotherm. The Redlich- Peterson isotherm
model combines elements from both the Langmuir and Freundlich equations [26] and the mechanism of
adsorption is a hybrid one and does not follow ideal monolayer adsorption
qe = KRCe /1+ aRCeß (4)
⧫Error analysis
The traditional approach of determining isotherm parameters is based on the linearized form of isotherm
equation by best fitting the linearized isotherm equation to the experimental data. However, the correlation
coefficient R2 generated from this method has the drawback that it may not provide the best isotherm constants
for correlating the original (non-linearized) isotherm equation with experimental data points. Because of the
inherent bias from linearization, alternative isotherm parameters are determined by non-linear approach. The
fitness of the isotherms, generated from non-linear approach, to the experimental data is optimized by error
analysis. In this project, sum of squared error (SSE) (Eq. (5)) is employed and the isotherm parameters are
determined by minimizing the SSE values across the entire concentration range of studies using the Solver add-
in with Microsoft’s spreadsheet, excel [22].
Where qeexp is the experimental value of qe (mg/g), qecal is the predicted value of qe by models (mg/g). N
indicates the number of data points in the experimental run
The amounts of adsorbed quantities of each dye at the equilibrium (qe), versus equilibrium dye concentration
were drawn in Fig. 2. The experimental adsorption obtained was compared with the adsorption isotherm models
and the constants appearing in each equation of those models were determined by non-linear regression analysis.
The results of this analysis are tabulated in Table 2. The square of the correlation coefficient (r2) are also shown
in this table.
Among the tested three-parameter equations, the better and perfect representation of the experimental results of
the adsorption isotherms is obtained using the Redlich Peterson and the Langmuir models (Fig. 2). According to
Table 2, the coefficients of correlation are near to 0.99.
The methylene blue and malachite green isotherms show an important adsorption with almost weak
concentration in the solution, it is an isotherm of the type H in Giles classification [27]. Generally, Isotherms
types H are the result of the dominance of strong ionic adsorbate-adsorbent interactions [28]. A chemical
adsorption of positively charged functional groups of MB and MG on the negatively charged surface groups of
the clays is proposed.
N
qqRMSE
ecale
)²(exp (5)
JMES, 2017, 8 (3), pp. 1082-1096 1087
The values of the maximum adsorption capacity obtained using the Redlich Peterson equation are near than
those calculated by the Langmuir and Freundlich models for the BR46 and the GM dyes, higher than the value
calculated for the BM dye. The mean value of the average percentage error for the studied compounds is for the
Redlich Peterson model than for the Langmuir and Freundlich models.
It can be also seen that the values of n at equilibrium are 4.04; 6.94 and 3.68 for the BR46, BM, and GM
respectively. It is noted that the values of n are bigger than 1, reflecting the favourable adsorption [29].
(b)
Figure 2: Adsorption isotherms of dyes (a): BR46 (b) BM and (c): GM (T °: (25 ± 2) °C, mclay: 400 mg /L) onto clay
JMES, 2017, 8 (3), pp. 1082-1096 1088
Modèlesd’isothermes BR46 BM GM
Langmuir
q0 (mg/g) 54.14 50,29 53,78
q0 (mmole/g) 0,12 0,16 0,12
KL (L/mg) 1,19 3,26 0,78
R² 0,99 0,96 0,98
RL 0,27 0,24 0,23
Δq % 4 3.8 7.3
Freundlich
KF (mg/g) 30.50 36.90 25.78
n 4.04 6.94 3.68
R² 0.96 0.95 0.97
Δq % 9 12.2 7.3
Redlich Peterson
KR (L.g-1
) 52.88 219.22 65.44
aR (L.mg-1
) 0.80 4.76 1.66
α 1.08 0.95 0.88
R² 0.99 0.96 0.98
Δq % 3.5 4.1 5.9
3.3. The temperature effect of BR46, BM and GM dyes onto clay
Evaluation of temperature was carried out with the scope of testing the ability of the clays in dyes removal in the
case of different kinds of effluents, bearing in mind the specific circumstances of dye stuff wastes.
The quantities of BR46, MB and GM adsorbed on the clay as function of solution temperature are shown in
Table 3. The table indicates that the increase in solution temperature increases the adsorbed quantities of BM
and GM, which may be due to increase in dye mobility that may occur at high temperature between the
adsorbent and the adsorbate [30]. However, the adsorbed quantity of BR46 decreases significantly with the
increase in temperature. From these results, the thermodynamic parameters such as the standard changes in free
energy (ΔG◦), enthalpy (ΔH◦) and entropy (ΔS
◦) were determined from the plot of log (KC) versus 1/T [31]:
Table 2:Isotherm parameters of the Langmuir, Freundlich and Redlich Peterson models obtained by using
non-linear regression
JMES, 2017, 8 (3), pp. 1082-1096 1089
Ln Kc = −ΔH°adsRT +ΔS°adsR (6)
Where R is the gas constant (8.314 J/ mol K), Kc is the equilibrium constant and T is temperature in K. The Kc
value is calculated from Eq. (6):
Kc = qe/Ce (7)
Where qe and Ce are the equilibrium amount adsorbed (mg/g) and equilibrium concentration (Ce in mg/L) at
different temperatures (mg/L).
The change in standard free energy (ΔG◦) can be calculated from the following equations:
ΔG°ads= ΔH°ads – TΔS°ads (8)
The slope and intercept of the plot gives the values of ΔH◦ and ΔS
◦. The temperature effect of the amount of
basic dye, BR46, adsorbed onto clay at various temperatures was examined. The results are shown in table 3. It
can be seen that the uptake of BR46 by clay decreases with increasing temperature, which indicates that the
adsorption of BR46 is controlled by an exothermic process, it is confirmed by the negative value of ΔH°. This
validates the subsistence of the attractive forces weakening between dye and adsorbent sites [32]. Similar trend
was observed for Chrysoidine R dye on fly ash [33]. The decrease in adsorption capacity of clay at higher
temperature may be attributed to the tendency of the dye molecules to escape from the solid phase to the bulk
phase with an increase in the temperature of the solution. Since the adsorption is an exothermic process, it
would be expected that an increase in solution temperature would result in a decrease in
, so the adsorption of BR46 onto clay and involves a physical adsorption [34].
For the BM and the GM dyes, the adsorption is endothermic in nature since the value of ΔH is positive. The
endothermic nature is also indicated by the increase in the amount of adsorption with temperature (Table 3)
[12]. The adsorption is associated with an increase in entropy of 37 J mol-1
K-1
and 50 J mol-1
K-1
for the BM
and the GM which shows that the adsorbed dyes molecules onto clay surface are organized in more random
fashion compared to those in the aqueous phase. Similar observations have been reported in the literature [35,
36]. The higher heat of adsorption obtained in this work indicates that chemisorption rather than the physical
adsorption is prevalent in this case [37].
An increase in adsorption with increase in temperature was observed, the process to be endothermic showing
that the chemical adsorption of the BM and GM onto. We note also that ΔH values obtained from adsorption of
BR46 onto clay are lower than that of BM and GM. This result for BR46 gives clear evidence that the
interactions between BR46 and the surface hydroxyl groups of clay may be weaker than those obtained for the
BM and the GM [38].
3.4. Activation parameters
With respect to the kinetic modelling, the pseudo-second order [29] model has been examined to find out the
adsorption mechanism:
Where qt is the amount of dye adsorbed (mg g−1
) at time t, qmax is the maximum adsorption capacity (mg g−1
),
and k2 is the rate constant of pseudo-second-order adsorption (g mg−1
min−1
). The straight-line plots of t/qt vs. t
for the second-order reactions for BR46, BM, and GM onto clay have also been tested to obtain rate parameters.
k, qmax, and the correlation coefficient r2 of dyes under different conditions were calculated from these plots. The
correlation coefficients (r2) found for the three dyes are 0.99. These results show that the adsorption system
belongs to the pseudo second-order kinetic model.
The values of the rate constant k2 at different temperatures listed in Table 5 were applied to estimate the
activation energy of the adsorption of BR46, BM and onto clay by the Arrhenius equation [39] as follows:
ln k2 = lnA – Ea/RT (10)
Where k2 is the rate constant of pseudo-second order adsorption (g/mg.min), A is the Arrhenius factor, R gas
constant (8.314 J mol-1
K-1
), T absolute temperature (K). Constants k2 were calculated for each temperature
(303; 313; 323; 333 K) (Table 4). The plot of Ln k2 versus 1/T for the adsorption of the dyes onto clay was
applied to obtain the activation energy, Ea from the slope, which used to determine the type of adsorption.
Activation energy for BR46 adsorption onto clay is 38 kJ/ mole. Its value between 5 and 40 kJ/mole is
corresponding to physical adsorption [40].The observed activation energy (Ea> 42 kJ/mole) indicated a
(9) tqqkq
t
t max
2
max2
11
JMES, 2017, 8 (3), pp. 1082-1096 1090
chemically controlled process [41] for the adsorption of BM and GM onto clay. These results are confirmed by
those of thermodynamic parameters found above.
Table 3: The thermodynamic parameters of BR46, BM and GM
BR46 (20 mg/L) BM (24 mg/L) GM (16 mg/L)
T (K) qe
(mg/g)
ΔG°
(kJ/mol)
ΔH°
(kJ/mol)
ΔS°
(kJ/mol K)
qe(mg/g)
ΔG°
(kJ/mol)
ΔH°
(kJ/mol)
ΔS°
(kJ/mol K)
qe(mg/g)
ΔG°
(kJ/mol)
ΔH°
(kJ/mol)
ΔS°
(kJ/mol K)
….. 45.68 -5.13 ….. ….. 35.06 -7.10 ….. …..
0.002 46.35 -5.50 6.081 0.037 35.39 -7.60 8.04 0.05
….. 47.10 -5.87 ….. ….. 35.72 -8.10 ….. …..
….. 47.96 -6.24 ….. ….. 36.22 -8.60 ….. …..
Table 4: Pseudo-second-order kinetic parameters for the adsorption of BR46, BM and GM onto clay at various
temperatures
Dyes T (K) qe (mg/g) K2 (g/ mg.min) R² E (Kj/mole)
BR46
303 40.54 0.09 0.99
39 313 39.65 0.11 0.99
323 39.36 0.26 0.99
333 38.91 0.29 0.99
BM
303 45.68 0.001
0.99
74
313 46.35 0.008 0.99
323 47.10 0.022 0.99
333 47.96 0.025 0.99
GM 303 35.06 0.013 0.99
64
313 35.39 0.039 0.99
323 35.72 0.072 0.99
333 36.22 0.14 0.99
3.5. Desorption studies
Wastewater streams containing dyes are generally loaded of salts, thus, the effect of salt electrolyte on BR46,
BM and GM removal needs to be investigated. To clarify the role of Na+
and Ca2+
ions on the desorption
phenomena, we have prepared different flask containing 50 mL of dyes clay solutions at 0.0006 M stirred
until equilibrium, at which were added 50 mL of solutions containing deionized water and different volumes
going from 2.5 to 50 mL of CaCl2 and NaCl separately taken from a stock solution of 0.003 M, so, a salt dyes
ratio going from 0.25 to 5. Generally, the presence of electrolyte such as NaCl, and CaCl2 may have different
effects in aqueous solution. These salts are dissociated in water to provide positive and negative ions (Na+, Ca
2+
and Cl-). The initial dyes concentration is 6.10
-4 M and the clay mass is 40 mg. The results showing calculated
percentage of qmax removal so the desorption study in the presence of calcium or sodium ions are shown in Fig.
3. This figure shows that there was a decrease in the adsorption capacities by increasing in the concentration of
salts added. However, as revealed in Fig. 3, there was a significant reduction in the adsorption of dyes so, an
increase in the desorption of these dyes onto clay when the salt dye ratio is less than 1. The percentage decrease
in adsorption capacity so, the percentage desorption studies was 18.8 %; 8.88 %; 10.18 % with NaCl solutions
and 33 %; 24.12 %; 22.18 % with CaCl2 solutions for BR46, BM and GM respectively. At low salt
concentrations, electrostatic repulsions are predominant so that high salt retentions are obtained. The retention
factor of NaCl and CaCl2 is low when the salts concentration is high with reduction in electrostatic interactions.
JMES, 2017, 8 (3), pp. 1082-1096 1091
This can be explained by the screening effect due to addition of NaCl or CaCl2. Theoretically, the electrostatic
forces between the adsorbent surface and adsorbate ions attractive and an increase in ionic strength will decrease
the adsorption capacity [42]. Whereas, when the salt dye ratio is more than 1, limited effect is occured (less than
10% inhibition) for the same salt concentrations about five times. This effect of ionic strength on dye ion
adsorption is often attributed to the competition between positives cations of the electrolytes (NaCl and CaCl2)
and dyes ions for the surface sites, where the cations of these salts solution compete much more effectively for
permanent negatively charged sites (silanol sites, Si-OH) on the kaolinite faces of the clay than on the aluminol
sites (Al–OH) [43].
-
-
- Figure 3: Na
+and Ca
2+effect on the dyes desorption: (a) BR46, (b) BM and (c) GM (T: 25 ± 2) °C,
m clay: 400 mg /L)
JMES, 2017, 8 (3), pp. 1082-1096 1092
It is also suggested that increasing salts dyes ratio can cause screening of surface negative charges by the
electrolyte salts ions leading to a drop in the adsorption of the dyes ions [44-45]. Therefore, a decrease in
adsorption of dyes, so, an increase in the desorption of them with increasing ionic strength of salts solutions
implies that increasing ionic strength is making the potential of the adsorbent surface less negative and thus
would decrease dyes ion adsorption [45, 46]. It is also observed that more dyes molecules ions were desorbed
from the adsorbent in presence of CaCl2 than dyes desorbed in the presence of NaCl (18.8 %; 8.88 %; 10.18 %
with NaCl solutions and 33 %, 24.12 %, 22.18 %) with CaCl2 solutions for BR46, BM and GM respectively
(Fig. 3); Moreover, The results of the dyes ions leached from the adsorbent demonstrate that the leaching
amount of divalent ions (Ca2+
) in all samples are significantly higher than that of univalent (Na+). Thus, the clay
surface is more enriched by the positives charges of the bivalent cations addition than by the monovalent ones
and make the repulsion between the adsorbent and the cationic dye external surface easy. The effect of Na+ and
Ca2+
on BM and GM dyes is nearly similar. As against, there is an increase in the desorption of BR46 (Fig.3),
this suggest that the interactions between BR46 and the surface hydroxyl groups of the clay may be more weak
than those between BM, GM and the clay, which is confirmed by the results observed for the thermodynamics
parameters and the activation energy.
3.6. IR adsorption mechanism of the BR46, BM and GM dyes.
ATR-FTIR spectra of the samples were recorded with a JASCO corp, FT/ IR 620 REV. 1.00. Spectrometer, as
presented in Fig. 4.
Figure 4: IR spectra of the BM dye, the raw clay and clay saturated with BM dye
JMES, 2017, 8 (3), pp. 1082-1096 1093
All spectra were taken in the range of 4000- 400 cm-1
. The Fig. 4 showed the clay, the dye and the clay saturated
with BM dye spectrums. The spectrum accorded to raw clay is demonstrated in section: 3.1. For the MB dye,
the peak related to the vibration of the aromatic ring at 1600 cm-1
was very prominent. Two peaks at 1495 and
1394 cm-1
were recognized as the C-N stretching vibrations. The vibrations of the CH3 group were found at
1359 cm-1
. The spectrum of MB adsorbed onto clay displayed in figure 4 presents a shift for the peaks
corresponding to MB with the aromatic ring vibration at 1604 cm-1
, the C-N stretching vibrations at 1490 cm-1
and 1390 cm-1
and the CH3 group vibrations at 1354 cm-1
, reflecting the evidence for the strong interaction
between MB and clay (by dipole - positive charge bond, hydrogen bond between the raw clay hydroxyl groups
and the free electron pair of the nitrogen in the BM molecule interactions respectively), in accordance with the
low values of RL obtained above.
Fig. 5 showed the clay, the dye and the clay saturated with GM dye spectrums. The figure representative of the
GM dye shows a predominant peak related to the vibration of the aromatic ring at 1591 cm-1
, the peaks related
to C-N stretching vibrations are seen at 1487 cm-1
and 1379 cm-1
. CH3 is observed at 1313 cm-1
, the spectrum of
GM adsorbed onto clay displayed in figure 5 shows a GM displacement of aromatic ring band vibration at 1610
cm-1
after adsorption, a shift for the C-N stretching at 1434 cm-1
and 1407 cm-1
, and a CH3 displacement of the
vibration band at 1344 cm-1
. This shows that the established interactions are dipole-dipole nature. For the tree
cationic dyes, the displacement is observed at near than about 1600 cm-1
, this indicates that COOH could be the
potential adsorption sites for interaction with the cationic dyes.
Figure 5: IR spectra of the GM dye, the raw clay and clay saturated with GM dye
JMES, 2017, 8 (3), pp. 1082-1096 1094
Moreover, we have observed for the BM and GM, an activation energy Ea> 42 kJ/mole indicating a chemically
controlled process [41]. This strong interaction was demonstrated by BM and GM desorption tests, we noticed
that these dyes have a very low desorption.
For the BR46 dye, clay saturated by dye shown in figure 6, we observe a predominant peak at 1600 cm-1
related
to aromatic ring, a peak at 1454 cm-1
related to C-N stretching vibrations and a peak at 1417 cm-1
for the CH3
group, Several weak peaks between 1253 cm-1
and 669 cm-1
were ascribed to the C-H in plane and out of plane
bending vibrations for the three dyes [47]. from this figure (Fig 6), we note a displacement of the vibration band
at 1610 cm-1
, a shift at 1358 cm-1
reflecting lower interaction BR46 dye molecules with clay. Also, the
activation energy observed for the BR46 is Ea ˂ 40 kJ/mole indicating a physical controlled process [41], this is
confirmed by desorption results (lower than BM and GM).
Finally, the cationic MB, BR46 en GM molecules were readily adsorbed onto negatively charged sites of the
clay by charge attraction. Therefore, it was concluded that electrostatic attraction might play a major role in the
initial bulk diffusion for the three dyes. This was in agreement with previous studies on the adsorption of acid
blue 40 onto P25 titania [37] and hexokinase onto silicon wafers [44]. They also indicated that the adsorption
was mainly driven by electrostatic forces. Similar to the reports in Ref. [46], where the MB- TNTs
nanocomposite was formed when MB was adsorbed onto TNTs.
Figure 6: IR spectra of the BR46 dye, the raw clay and clay saturated with BR46 dye
JMES, 2017, 8 (3), pp. 1082-1096 1095
Conclusion
The results in this study showed that the low cost and the abundant Moroccan clay can be used as an adsorbent
for the removal of BR46, BM and GM from aqueous solution, a particular attention was paid to competitive
adsorption and the effect of inorganic ions.
The sorption data indicate that Redlich Peterson equation provides better fit than Langmuir and Freundlich
equation. The kinetic experiments imply that sorption process obeys the pseudo-second-order kinetic model for
all pollutant studied. The temperature effect was also used to calculate the thermodynamic activation enthalpy,
entropy, and free energy of adsorption. The negative value of ∆G° confirms the spontaneous nature adsorption
process. The positive value of ∆S° shows the increased randomness at the solid-solution interface during
adsorption. The positive value of ∆H° for GM and BM dyes indicated that the adsorption process is
endothermic; the chemical nature of adsorption is occurred consequently. Whereas, it is negative for the BR46
dye, indicating the exothermic and physical processes nature. The activation energy of adsorption of BR46. GM
and BM onto clay is found to be 40; 64 and 76 kJ/mole, respectively. This confirms the results found by the
thermodynamics parameters (physical nature for BR46, and the chemical one for GM and BM).
The role of Na+
and Ca2+
ions on the desorption phenomena of the BR46, BM and GM dyes was studied by
adding to different flask containing 50 mL of dyes clay solutions at 6. 10-4
M stirred until equilibrium, 50 mL of
solutions containing deionized water and different volumes of CaCl2 and NaCl separately taken from a stock
solution of 0.003 M with a salt dyes ratio going from 0. 25 to 5. The results show that there is a decrease in the
adsorption capacities by increasing in the concentration of salts added. However, a significant increase in the
desorption of these dyes onto clay when the salt dye ratio is less than 1 was observed. Whereas, when the salt
dye ratio is more than 1, limited effect is occured (less than 10% inhibition) for the same salt concentrations
about five times. It is also observed that more dyes molecules ions were desorbed from the adsorbent in
presence of CaCl2 than dyes desorbed in the presence of NaCl. These results have been confirmed by IR spectra
decomposition, adsorption mechanism can be ascribed to chemical sorption for the BM and the GM, physical
for the BR46 and electrostatic attraction might be predominant in the initial bulk diffusion.
In conclusion, Moroccan clay of the Safi adsorbent shows:
- A good potential for the removal of cationic dyes.
- Its use as an adsorbent for the wastewater treatment loaded with textile dyes is promoted by its natural
abundance and ease of use.
- A low desorption percentage promoting the use of clay saturated with dyes in the ceramics industry and
decoration products, so, an increase and a development of the Safi city economy.
Acknowledgements: We thank all the staff and professors for all the effort they made to achieve this work.
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